Membrane receptor proteins may be targets for therapeutic intervention and therefore physical characterization and elucidation of receptor structure are important solubilization and purification of the receptor proteins are essential steps in physical characterization. A major drawback to physical characterization and purification has been the difficulty in solubilizing the proteins in a sufficiently stable and active form.
The use of detergents to solubilize receptor proteins has been employed in either pre- or post-ligand binding. However, problems have been associated with both procedures. When the protein has been solubilized prior to ligand binding, protein stability is decreased and recovery is typically low (in the order of about 5%). Where the protein has been solubilized after ligand binding, protein stability is increased (to about 50%) however, the binding analysis of the receptor protein is not possible. Therefore a method for solubilizing desired proteins while maintaining both stability and the possibility of performing binding analyses is needed.
The AII receptor represents a potential site of intervention in the development of new drugs for therapy of hypertension. However, purification of the AII receptor has proven technically difficult. The inability to solubilize sufficient amounts of AII receptor protein from cell membranes in a form capable of binding radioligand has hindered progress in this area. A primary problem in this regard has been the rapid loss of binding after solubilization of the AII receptor with ionic and nonionic detergents. More recently, the zwitterionic detergent 3-[(3-cholamidopropyl)-dimethylammonio]1-propanesulfonate (CHAPS) has been used to solubilize AII receptors from the bovine adrenal and rat adrenal glomerulosa. However, specific [.sup.125 I]-AII binding to CHAPS solubilized protein was labile with a 50% loss occurring in 10 days. In order to overcome these limitations, membranes have been solubilized from target tissues after photoaffinity labeling of AII receptor sites. A drawback to this approach is that prelabeling of membranes with photoaffinity ligands typically results in high nonspecific binding and precludes the evaluation of pharmacological characteristics of the soluble receptor. Despite this recent progress in the development of photoaffinity ligands and advances in small scale purification of the AII receptor, this protein has not been solubilized in a form which allows full pharmacological evaluation or which leads to purification. Therefore, there is a need for a method which will solubilize membrane proteins, including AII receptors in a form suitable for pharmacological characterization and biochemical purification.